Development and extraction of hydrocarbon reserves involves the collection and analysis of extensive data pertaining to fluids in the geological formations. For example, economic evaluations of hydrocarbon reserves in geological formations involve a thorough analysis of the formation fluids. Similarly, development and production considerations, such as methods of production, efficiency of recovery, and design of production systems for the hydrocarbon reserves, all depend upon accuracy in initial and continuing analyses of the nature and characteristics of reservoir hydrocarbon fluids. Formation analysis and evaluation requires constant measurements of formation fluids to acquire data with respect to fluid properties.
Determination of formation fluid characteristics, such as density, viscosity, temperature, pressure, gas-oil ratio (GOR), bubble point, among others, provides a way to analyze the nature and characteristics of a reservoir formation. Measurements of formation fluid properties yield insight into geological formations, such as permeability and flow characteristics. The data also provide a way to assess the economic value of hydrocarbon reserves.
Typically, formation fluid samples are obtained during the exploration phase of oilfield development, and the thermophysical properties of the fluids are determined at the surface. However, often it is necessary and/or desirable to determine certain reservoir fluid properties, such as density and viscosity of crude oil or brine, at the pressure and temperature of a hydrocarbon reservoir. Although the pressure and temperature of fluid samples at the surface can be adjusted to the conditions in the reservoir, it is sometimes difficult to obtain a fluid sample at the surface that closely replicates the downhole formation fluid in chemical composition.
It has been found that variations tend to occur in the extracted fluid samples due to volatility of lighter hydrocarbons, deposition of solids, contamination by drilling fluids, and so on. Moreover, it is very expensive to extract downhole fluid samples from a borehole, and to maintain and handle the extracted fluid samples at the surface under downhole pressure and temperature conditions. It is advantageous, therefore, to acquire and transmit fluid properties data downhole for the data to be analyzed at the surface, thereby significantly reducing the time and costs associated with hydrocarbon reservoir analysis and evaluation.
Answer products, such as analyses based on downhole fluid analysis, that relate to reservoir production and optimization are typically based on analyzing extremely small samples of downhole fluid, i.e., by volume relatively less than 10−9 of the hydrocarbon reserves in a typical geological formation. Moreover, the composition and characteristics of formation fluids in a reservoir are subject to change as the hydrocarbon reserves are developed and extracted. Therefore, it is advantageous to regularly monitor formation fluid properties by taking frequent downhole measurements of formation fluids throughout the exploration and production phases of an oilfield.
The oilfield fluids typically handled in the oil exploration and production industries are an extremely harsh operating environment in comparison with the customary conditions where small measuring and data collection devices, such as microchip sensors, are used. For example, typical downhole fluid conditions in producing hydrocarbon reservoirs include downhole temperatures from 50 to 175 degrees Celsius or more, downhole pressures from 100 to 2,000 bar, densities in the range 500 to 1300 kg m−3, and viscosities from 0.1 to 1000 mPa s.
As a result of their chemical and compositional properties, oilfield fluids tend to be erosive and corrosive in nature. Due to the difficult environments in which oilfield equipment is deployed, the equipment must be capable of withstanding severe shock and corrosion due to the possibility of corrosive fluid constituents, such as H2S and CO2, and solid particulates, such as sand, being present in flowing formation fluids. Reference is made to J. A. C. Humphrey, Fundamental of Fluid Motion in Erosion by Solid Particle Impact, Int. J. Heat and Fluid Flow, Volume 11, #3, Sep. 3, 1990, and references therein, for a discussion on erosion that is caused by solid particulates, such as sand, in fluids.
Furthermore, hydrocarbon reservoir fluids tend to be complex and may contain chemical components ranging from asphaltenes and waxes to methane. The composition of hydrocarbon fluids makes deposition of waxy materials on downhole tools a distinct possibility, which often is a cause of fouling of the tools.